This actually may have a real benefit to some specialty folders - such as junk/nojunk folders that are looked for by a scanning re-classifier. Can you give an example of how to make...Inbox/Junk non-deletable (or even better, update the wiki)?
INBOX is special-cased in cmd-delete.c.I don't yet see where plugins get invoked so I haven't yet found where the ACL plugin can veto the delete command.